While adjusting the amount of enemies in a level is a great execution: Croteam missed 2 things when it comes to official levels:

1. A lot of gnaars and beheaded soldiers were replaced by kleer skeletons, often reducing the enemy variety. I think it would have been better if they had used more beheaded soldiers and gnaars in tandem with kleer skeletons. While gnaars and beheaded soldiers in the late game are rather weak compared to weapons you'll have, adding them in battles often brings it subtle changes, forcing you to prioritize the targets a bit more.

2. They gave you double the ammunition compared to normal, hard and mental. While the amount of enemies is larger than in normal difficulty, your ammo supplies are still enough to take down every enemy in the game with some ammo to spare if you're careful. As you have plenty of ammo, you need think less about how you spend your ammunition.
Agree or disagree?

About 1), I think that is kind of the point. Same has been done since Wolf 3D and it worked.

Agree with number 2. The double ammo for hardest difficulty has survived since Doom, but that was because enemies respawned. Unfortunately, in Serious Sam, it makes ammo management on serious irrelevant. If a game is balanced for a certain number of enemies, there is 0 reason to give the player double ammo.

I think that Mental difficulty was done wrong. It being based on hard and not serious basically makes it easier than serious by quite a margin. And you are rewarded with more score. The phasing of enemies doesn't matter much (at least to me), but I feel the reduced damage received and number of enemies all the time compared to serious.

I just finished my Serious Sam: TFE video walkthrough on Serious and can state with ease that they absolutely nailed the difficulty. It's always challenging and extremely fun to play, particularly when some time has passed since your last playthrough, since you remember most things, but more "specific" enemy spawns are a great test of one's reaction speed and accuracy.